The present invention relates to an exhaust gas recirculation system for controlling the flow of exhaust gas from an exhaust gas passage to an engine intake passage of an internal combustion engine. More particularly, the present invention relates to control logic for an electrical actuator of such a system.
Exhaust gas recirculation (EGR) systems are employed in automotive vehicles in order to help reduce various engine emissions. Such systems typically employ a EGR valve that is disposed between the engine exhaust manifold and the engine intake manifold, and operable, when in an open position, to recirculate exhaust gas from the exhaust side of the engine back to the intake side.
An actuator is employed for moving the EGR valve between its open and closed positions, with the recirculation of exhaust gas being appropriate only at certain times. Typically, the appropriate time for recirculation is when the vehicle is traveling at lower speeds, when the pressure at the intake side of the engine is less than the pressure at the exhaust side of the engine.
Many prior art EGR systems employ air pressure as an actuator. However, in order to achieve more precise control, electrically-actuated EGR valves have also been introduced. One such electrically-actuated EGR valve is disclosed in pending U.S. application Ser. No. 881,622, filed Jun. 24, 1997, that is assigned to the present assignee, the disclosure of which is incorporated herein by reference.
Electrically-actuated EGR valve systems may employ software-implemented control logic which implements closed loop control. Such control logic controls current to an electric actuator motor which, in turn, positions the EGR valve. In such systems, the control logic may generate pulse width modulated (PWM) signals to power the actuator motor, and modulate the acceleration and deceleration of the EGR valve as it moves to its desired positions.
With respect to the closing of an electrically actuated EGR valve, prior art systems have implemented control logic that, once the valve is brought to a completely closed position (as detected by a valve position sensor), reduces the PWM signal to a nominal low value, resulting in a low holding current through the actuator motor for holding the valve in the closed position. This closing control logic has several disadvantages. First, when the valve is in the closed position and a low holding current (corresponding to a nominal PWM signal) is applied to the actuator motor, even moderate differential pressures across the valve can unseat the valve, causing a recirculation of exhaust gas at an improper time until the control logic senses the problem and increases the PWM signal to reseat the valve. Second, the prior art control logic does not take into account situations where the valve position sensor is in error. If the position sensor is out of adjustment, even by only a small amount, the sensor can improperly signal the control logic that full current is needed in order to close the valve when, in fact, the valve is already seated. This continuous application of full current when the valve is already seated creates a motor overcurrent condition that can ultimately result in motor failure.